Whether you work in a fleet operation or for a garage subcontracting HGV repairs and servicing, you might be familiar with the following:
- Repeated early failure of one battery in a 24-volt pair
- Vehicles that test fine one week and flag low voltage the next
- Alternators and chargers blamed, replaced, then blamed again.
And now, because you’re here, you’re wondering if battery balancing can fix that uneven discharge. The straightforward answer is yes, but it’s more about management than an outright ‘fix’. The truth is, there’s no practical way to completely prevent imbalance, so battery balancing is the next best thing.
Why do 24-volt battery systems become unbalanced?
As you’re aware, the 24-volt system in most buses, HGVs and so on relies on two 12-volt batteries wired in series. These batteries may appear identical (same brand, model and specs), but they contain microscopic differences.
Over months of use, those tiny differences become increasingly pronounced due to natural wear and tear. In the electrical system, that refers to every discharge and recharge, but cold starts, auxiliary loads, idle drains and short run cycles can also have a huge impact.
Alternators charge ‘the 24-volt system’ as a whole, as opposed to each 12-volt battery individually. That’s because they’re wired in series. Once one battery pushes the system toward its regulation voltage, the alternator, in effect, backs off, leaving the other battery undercharged.
Eventually, these two 12-volt batteries start behaving slightly differently. At rest, you or your technicians might not notice any problems because the overall circuit may have a normal resting voltage. But dig a little deeper with load testing. If the batteries are imbalanced, you’ll find that one unit is under more stress thanks to voltage drop, heating and depth of discharge relative to remaining capacity, even though both carry the same current in series.
Chemical effects of 24-volt battery imbalance
During discharge, the weaker battery reaches low terminal voltage first because it has less usable capacity and a poorer voltage response under load. As state of charge (SoC) falls, electrolyte concentration near the plates drops and the internal resistance increases. The stronger battery might still have capacity, but the system has to follow the lowest voltage in the series circuit. Because the series circuit follows the lowest voltage in the chain, discharge must stop early. So, you effectively ‘lose’ all the usable capacity in the stronger battery.
When your batteries recharge (either thanks to the alternator or from 24-volt battery charging equipment), we see the opposite problem. The stronger battery reaches a higher SoC and higher terminal voltage first. The weaker one never reaches full charge, leaving lead sulphate on the plates. Continued cycles like this increase and harden the sulphate crystals, reducing the active surface area and resulting in less capacity.
It’s a vicious cycle, with both batteries struggling more until, sooner or later, something gives and you end up with a breakdown on your hands.
As we mentioned earlier, there’s really nothing we can do about this. Preventing battery imbalance is all but impossible, short of unrealistically precise control over manufacturing, temperature and charging at a chemical level. That, of course, is not usually feasible. And while you could replace the batteries, the same imbalance will reoccur.
However, thanks to battery balancing, you won’t have to spend your days worrying about 24-volt battery imbalance.
What is battery balancing?
Battery balancing is a straightforward procedure with the right equipment. It’s repeatable and easy work for any trained 24-volt technician or mechanic. Here are the practical steps involved, and why it’s worth considering for your workshop:
- Test the system – use a specialist battery tester designed for balancing batteries. Your technician tests each battery individually, still in circuit where possible. Tests should include standard voltage tests, but also estimated SoC, internal resistance, and load testing.
- Balance the batteries – set the settings and let the process get started. Your equipment automatically rebalances the two 12-volt batteries, ensuring they’re as similar in SoC as possible, ready for series work in your 24-volt vehicles.
- Retest the system – once balancing has finished, run a couple more tests to check everything looks okay. Your vehicle’s batteries are now balanced.
- Develop a maintenance schedule – make sure your technicians test and balance the 24-volt system at every regular service. It’s such a simple and cost-effective operation compared with the headache of a non-start breakdown.
Most good 24-volt battery balancers, such as the Midtronics CX PRO 100-2, don’t require removing the batteries from the vehicle. Some, however, may.
Why does 24-volt battery balancing work?
The gradual imbalance that develops in a series system means one battery is overworked while the other is underworked. Both struggle. Battery balancing works because it makes the two batteries behave in a very similar chemical way, correcting the mismatch described earlier.
When both 12-volt batteries are at the same SoC and, ideally, state of health (SoH), the plate reactions occur at similar rates in each. Thus, the 24-volt system in your vehicles can perform much better, with a significantly reduced risk of breakdowns.
SoC alignment ensures neither battery becomes the limiting low-voltage unit under load or forces early voltage regulation during charging. Also, under load, similar internal resistances and voltage drops mean one battery doesn’t drag the whole 24-volt system down with it.
Overall, you keep usable capacity available and stabilise the entire system voltage. You also stop one battery ageing faster than the other. In turn, that means fewer battery and electronic problems in your fleet, which keeps vehicles on the road, reduces the high costs of breakdowns and supports proactive maintenance with more predictable workshop planning.
Here at Rotronics, we stock a wide range of battery testers and chargers designed to help you find and correct those imbalanced 24-volt systems. Check out the Midtronics CX PRO 100-2 or get in touch with our team to learn more.
